In recent years, there has been a remarkable development in the technique of an optical pickup apparatus to reduce the wavelength of the laser optical source used as an optical source to record and/or reproduce information on and/or from an optical disk. For example, successful efforts have been made to commercialize a laser light source with a wavelength of 400 through 420 nm including a blue-violet semiconductor laser and a blue SHG laser for converting the wavelength of an infrared semiconductor laser using the second harmonic. When using an objective lens having the same numerical aperture (NA) as that of the DVD (Digital Versatile Disk), use of the blue-violet laser light source allows about 15-GB information to be recorded on an optical disk having a diameter of 12 cm. When the NA of the objective lens is increased to 0.85, about 25-GB information can be recorded on the optical disk having a diameter of 12 cm. In the following description of this Specification, the optical disk using the blue-violet laser light source and magneto-optical disk will be collectively called the high-density optical disk.
In the high-density optical disk using an objective lens having an NA of 0.85, there is an increase in the coma aberration caused by the inclination (skew) of the optical disk. To cope with this problem, the thickness of the protective layer is designed smaller than that in the case of a DVD (0.1 mm as compared to 0.6 mm in the case of DVD) to reduce the coma aberration caused by skew. Incidentally, the high-density optical disk of this type capable of properly recording and/or reproducing information is not sufficient to win the fame of having a commercial value of the optical disk player/recorder (optical information recording/reproducing device). In view of the fact that the DVD and CD (Compact Disk) for recording a great variety of information are available on the market at present, the high-density optical disk of this type capable of properly recording and/or reproducing information itself is not sufficient. For example, to enhance the commercial value of the optical disk player/recorder for a high-density optical disk, it will be necessary to ensure that the DVD and CD (Compact Disk) owned by a user can also properly record and/or reproduce information. Against this backdrop, the optical pickup apparatus mounted on an optical disk player/recorder for a high-density optical disk is required to ensure the capabilities of properly recording and/or reproducing information on and/or from a high-density optical disk, DVD and CD interchangeably.
One of the possible methods for ensuring the capabilities of properly recording and/or reproducing information on and/or from a high-density optical disk, DVD and CD interchangeably is to switch between the optical system for high-density optical disk and the optical system for DVD or CD on a selective basis in response to the recoding density of the optical disk that records and/or reproduces information. This will require a plurality of optical systems and will make it difficult to design a compact configuration of an optical system. This will also increase the production cost.
To ensure the simplified structure and reduced costs of an optical pickup apparatus characterized by compatibility, the optical system for high-density optical disk and the optical system for DVD or CD are preferably integrated for common use so as to minimize the number of the optical components constituting the optical pickup apparatus. Integrating the objective lenses arranged oppositely to the optical disk for common use is the best way to ensure the simplified structure and reduced costs of an optical pickup apparatus. To get a common objective lens for the optical disks of various types having different wavelengths for recording and/or reproduction, an objective optical system must be provided with an optical path difference giving structure wherein spherical aberration depends on wavelength. This will reduce the spherical aberration caused by the differences in wavelength or thickness of the protective layer.
Patent Literature 1 discloses an optical pickup apparatus provided with an optical path difference giving structure and an objective lens that can be used commonly for both high-density optical disk and conventional DVD and CD.
Patent Literature 1: Specification of International Publication Number 2007/102315